This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the presently described embodiments. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present embodiments. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Drilling and production operations for the recovery of offshore deposits of crude oil and natural gas are taking place in deeper and deeper waters. Drilling and production operations in deeper waters are typically carried out from floating vessels rather than from stationary platforms resting on the ocean floor and commonly used in shallow water. According to conventional procedures, a vessel is dynamically stationed, or moored, above a well site on the ocean floor. After a wellhead has been established, a blowout preventer (“BOP”) stack including one or more BOPs is mounted on the wellhead to control the pressure in the wellhead.
Typical BOPs are used as a large specialized valve or similar mechanical device that seal, control, and monitor oil and gas wells. The two most common categories of BOPs are rain BOPs and annular BOPs. BOP stacks frequently utilize both types of BOPs, typically with at least one annular BOP stacked above several rain BOPs. The annular unit or units allow for sealing off an annulus between a tubular in the BOP bore (e.g., drill pipe) or on an open hole. The rain units in rain BOPs allow for shearing drill pipe in the case of shear rams, sealing off around drill pipe in the case of pipe rains, and sealing the BOP bore in the case of blind rams. Typically, a BOP stack may be secured to a wellhead and may provide a safe means for sealing the well in the event of a system failure.
FIG. 1 shows a prior art annular BOP 100. Annular BOP 100 comprises a vertical bore 102 extending through a housing 104 and disposed about a longitudinal axis 106. A packing element 108 is disposed within the annular BOP 100 about the longitudinal axis 106. The packing element 108 includes an annular elastomeric body 110 and a plurality of inserts 112. The inserts 112 are distributed radially about the longitudinal axis 106. The packing element 108 includes a bore 114 concentric with the vertical bore 102 of the annular BOP 100.
The annular BOP 100 is actuated by pumping a fluid into a close chamber 116 to apply pressure to a piston 118, thereby moving the piston 118 upward. As the piston 118 moves upward, the piston translates force to the packing element 108. The force translated to the packing element 108 from the piston 118 is directed upward toward an inner surface 120 of the annular BOP 100 and inward toward the longitudinal axis 106 of the annular BOP 100.
Because the packing element 108 is retained against the inner surface 120 of the annular BOP 100, the packing element 108 does not displace upward from the force translated by the piston 118. Rather, the packing element 108 displaces inward from the translated force, which compresses the packing element 108 toward the longitudinal axis 106 of the annular BOP 100. In the event a drill pipe is located within the annular BOP 100, with sufficient radial compression, the packing element 108 will seal about the drill pipe into a closed position. In the event a drill pipe is not present, the packing element 108, with sufficient radial compression, will completely seal the bore 102.
The annular BOP 100 goes through an analogous reverse movement when fluid is pumped into an open chamber 122. The fluid translates downward force to the piston 118, such that the piston allows the packing element to radially expand to an open position. The annular BOP 100 can be cycled between the open and closed positions as necessary.
When run into the closed position, the annular BOP 100 seals off only on the pressure below the annular BOP 100 by creating a sealing point around the elastomeric body 110 of the packing element 108. Because of the geometry of the annular BOP 100 and its packing element 108 as well as the distribution of inserts 112 about the packing element 108, the annular BOP 100 is not able to seal off pressure from above the annular BOP 100. That is, pressure from above the annular BOP 100 can access the elastomeric body 110 of the packing element 108, thereby causing it to extrude. To overcome this problem, operators may include a plurality of annular BOPs in a single BOP stack to ensure sealing above and below the BOP stack. However, inclusion of additional annular BOPs, including additional housings, packing elements, pistons, etc., adds undesirable height to the BOP and is costly.
Accordingly, an annular BOP capable of sealing off pressure from above and below the annular BOP is desirable.